Trying to update my GUI from remote object but getting Exception

I'm trying to make some basic chat client/server application using .net remoting.I want to make windows form client application and console server application as separate .exe files. Here im trying to call server function AddUser from client and i want to AddUser function update my GUI (richtexbox). When i try to do that i get exception: "[italic]SerializationException: Type Topic in Assembly is not marked as serializable[/italic]".

This will work if i use string variable instead of GUI RichTextBox control which im trying to update from another thread. Im begginer with .net remoting and any suggestion how ti fix this is welcome.

Topic class supposed to be Marshal By Reference. Marking Topic class as Serializable does not change anything as its already inherited from MarshalByRefObject class. Even if i change still get same error.